The antithesis of what a good boy should be - rude, obnoxious, violent and stupid. Formed by spelling 'boy' bacwards, it was coined in England in the 18th century as it was very popular amongst upperclasses to speak backwards at the time.

"Yobbo or yob is a slangterm for an uncouth or thuggish working-class person. The word derives from a back slang reading of the word "boy" (boy or boyo reversed becomes yob or – slightly modified – yobbo)."